Devices of this kind are already known, in which the semiconductive organic material consists either of a monomer organic substance, constituted by fluorescent molecules, such as anthracene, perylene, and coronene, or by molecules of an organic dye, or of a conjugated organic polymer such as poly-(p-phenylene-vinylene).
In these devices, the electrode which emits electrons is, for example, constituted by a layer of a metal chosen from among aluminium, magnesium, and calcium, or by a layer of metallic alloy, such as an alloy of magnesium and silver, and the electrode which emits holes is constituted, for example, by a layer of a metal such as gold or by a layer of tin oxide (SnO.sub.2) or mixed indium and tin oxide (ITO).
Such a device is described in the international patent application published under number WO 90/13148.
Such electroluminescent devices can be used in particular as light-emitting diodes in display elements, as well as for the manufacture of flat screens for portable computers or television sets.
These devices feature the advantage of easily allowing for the manufacture of large display surfaces, as well as allowing for an adjustment of the emitted light wavelength, therefore the emission colour, by selecting in an appropriate manner the semiconductive organic material which constitutes the electroluminescent layer from among the large number of known materials, which are suitable for this purpose, as well as the multiple combinations or modifications of these materials which are available to specialists.
In addition to this, these devices, in general, have a light emission efficiency which is quite acceptable, within the current state of the art, and which seems to be susceptible to improvements in the future within the capability of persons skilled in the art.
According to the prior state of the art relating to devices of this kind, the hole-injecting electrode has been provided in the form of a transparent layer, constituted, for example, by a mixed indium and tin oxide, the electron-injecting electrode itself being opaque or reflective. Devices of this type can emit light on only one face. According to one of the variants of the device described in application WO 90/13148, it is however mentioned that at least one of the charge-injecting contact layers, if these layers are of gold or aluminium and do not exceed a certain thickness, is transparent or semitransparent. It is not however specified which of these layers is transparent or semitransparent.
In addition to this, the devices currently known feature the disadvantage that they have a too short lifetime in regard of the envisaged industrial applications. More specifically, the best known devices of this type, in which the electroluminescent organic material layer is constituted by a monomer organic substance, only allow for a maximum period of use of the order of a thousand hours, in continuous operation, while the best known devices, in which the electroluminescent organic material layer consists of a conjugated polymer, do not in general resist a period of continuous operation greater than about a hundred hours.